Conventional cryogenic air separation processes normally involve at least two fractional distillation columns: a "low pressure" column, from which is withdrawn fluid oxygen bottom product of specified purity plus gaseous nitrogen overhead product, plus a "high pressure rectifier" which receives the feed air, provides a reboil to the LP column and LN.sub.2 reflux for both columns by indirect exchange of latent heat between the two columns, and provides oxygen enriched liquid air bottom product (kettle liquid) which is subsequently fed to the LP column.
The conventional flowsheets provide the bulk of the refrigeration necessary for the overall separation process in either of two conventional manners: by work expanding either part of the HP rectifier overhead nitrogen to exhaust pressure (slightly below LP column overhead pressure), or expanding part of the feed air to LP column intermediate height pressure. U.S. Pat. No. 3,327,488 illustrates the above two approaches in the same flowsheet, although for economic reasons usually only one or the other is used.
The refrigeration compensates for heat leaks, heat exchanger inefficiency, and other effects. Even with the most modern and efficient expanders, there is still required an expander flow of between about 8 and 15% of the inlet air flow to provide the necessary refrigeration, dependent on the size and design of the separation plant. This flow represents a loss of process efficiency, which can be manifested in various ways: lower recovery and/or purity of oxygen than would otherwise be possible; lower recovery and/or purity of coproduct argon; more machinery (and capital cost) to achieve acceptable recoveries and purities; or lower O.sub.2 delivery pressure than would otherwise be possible.
The conventional refrigeration techniques have no beneficial effect on the efficiency of operation of the fractional distillation columns. Column efficiency is indicated by closeness of operating and equilibrium lines, that is by the absence of mixing of streams of greatly differing composition and/or temperature. A new refrigeration technique which interacts with the distillation columns so as to make them operate more efficiently would be advantageous even if the refrigeration technique per se were no more efficient than the conventional refrigeration techniques. That is one objective of the present disclosure.
One alternative means for providing refrigeration has been disclosed in the prior art which beneficially interacts with a distillation column. As disclosed in U.S. Pat. No. 4,543,115 and British Patent No. 1271419, it is possible to work-expand part of the supply air to an intermediate pressure which is sufficiently higher than LP column pressure such that the partially expanded air will exchange latent heat with LP column intermediate height liquid in an intermediate reboiler. The air totally condenses while providing intermediate reboil to the LP column. The liquid air is subsequently depressurized to LP column pressure and fed to a height above the intermediate reboiler height. The LP column operates more efficiently due to the intermediate reboil below the feed point. On the other hand, the reduced pressure ratio of expansion of the refrigeration air necessitates a greater mass flow to obtain a given amount of refrigeration. Thus more air bypasses the HP rectifier, and the separation is made more difficult. This effect largely negates any benefit from improved LP column efficiency. Overall, the absence of a net thermodynamic benefit coupled with the disadvantage of requiring an additional item of capital equipment (the intermediate reboiler) have prevented this refrigeration technique from being used in any known application during the 15 years it has been publicly disclosed.
What is needed, and one objective of this invention, are improvements to this mode of refrigeration for an air separation process which further improve column operating efficiency, which reduce the amount of air required to be fed to the expander, and which reduce the amount and cost of added capital equipment. Furthermore, it is desired to obtain these improvements in flowsheets for the production of high purity oxygen plus coproduct argon, in addition to the more conventional flowsheets for the production of low to medium purity oxygen or high purity nitrogen.
It is known to reboil the bottom of the LP column by latent heat exchange with any of three gases: HP rectifier overhead N.sub.2 ; partially condensing feed air (U.S. Pat. Nos. 3,113,854, 3,371,496, 3,327,489, 3,688,513, and 4,578,095); or totally condensing feed air (U.S. Pat. No. 3,210,951, 4,208,199, and 4,410,343). Similarly it is known to evaporate the liquid oxygen bottom product of the LP column to gaseous oxygen product by latent heat exchange with any of the same three gases. U.S. Pat. Nos. 3,113,854, 3,371,496, 3,327,489, and 4,560,398 disclose partial condensation LOXBOIL, while U.S. Pat. Nos. 3,210,951, 4,133,662, 4,410,343, and 4,507,134 disclose total condensation LOXBOIL.
U.S. Pat. Nos. 3,210,951 and 4,410,343 both show a single heat exchanger in which about 40 to 56% of the feed air is totally condensed to provide both LOXBOIL and LP column reboil, and then the liquid air is divided and fed to both columns.
When the LP column is reboiled by HP rectifier N.sub.2, whereas LOXBOIL is via condensation, the LOXBOIL pressure is somewhat higher than the LP column bottom pressure. Although that pressure increase could be accomplished by a liquid oxygen pump, a preferred method is to use the barometric or hydrostatic head of a column of liquid oxygen, i.e., boil the LOX at a suitably lower elevation than the LP column bottoms reboiler. This is disclosed in U.S. Pat. Nos. 4,133,662, 4,507,134, 4,560,398, and South African application No. 845542 dated July 18, 1984 filed by Izumichi and Ohyama.
It is known to depressurize the HP rectifier bottom liquid (kettle liquid) and then reflux the HP rectifier by exchanging latent heat between HP rectifier overhead vapor and the depressurized kettle liquid. The evaporated kettle liquid is then fed to the LP column. This is disclosed in U.S. Pat. Nos. 4,410,343, 4,439,220, and 4,582,518.
It is known to apply the work developed by the refrigeration expander toward additional warm end compression of part of the compressed air supply. The further compressed air may then be used for conventional refrigeration (German patent application No. 845542 published 06/19/80 and filed by Rohde), or for TC LOXBOIL (U.S. Pat. No. 4,133,662, USSR Pat. No. 756150, and South African application No. 2854508 (supra)).
When adding intermediate reflux to a distillation column, the initial amount added allows a virtually one-for-one reduction in the overhead reflux (for specified recovery and purity). The benefit from intermediate reflux continues to increase as more is added until a "pinch" is reached: the operating line closely approaches the equilibrium line. Further additions of intermediate reflux beyond that point decrease the benefit, i.e., provide no more decrease in the amount of overhead reflux required. For an air separation process wherein liquid air is used as intermediate reflux, the optimal amount of liquid air reflux is about 5 to 10% of the feed air, for both the LP column and the HP rectifier. Greater liquid air flow rates do not provide any further decrease in the overhead reflux requirement.
Thus in summary air partial expansion refrigeration as disclosed in the prior art causes (1) an advantageous reduction in the LP column overhead reflux requirement; (2) no change in the HP rectifier overhead reflux requirement; (3) a disadvantageous decrease in the reboil through the HP rectifier and the lower section of the LP column; and (4) a disadvantageous increase in capital equipment.
The combination of AIRPER refrigeration plus warm companding of the refrigeration air was disclosed by applicant in U.S. Pat. No. 4,670,031, which is incorporated by reference.
Fractional distillation refers to the process of separating a mixture of two or more volatile substances into at least two fractions of differing volatility and composition by countercurrent vapor-liquid contact whereby a series of evaporation and condensations occur. "Intermediate height" of a fractional distillation columns signifies a location having countercurrent vapor-liquid contact stage(s) both above and below it.